The present invention provides a lithium halide primary cell having a shelf life extendable to more than ten years by utilizing a pelletized depolarizer consisting of P2VP+P2VP.n.sub.1 +n.sub.2 I.sub.2 wherein n.sub.1 &gt;0 and n.sub.1 +n.sub.2 is equal to 3 to 30 (parts of I.sub.2 for each part of total organic). Preferably n.sub.1 n.sub.2 is equal to 15 to 26 parts of I.sub.2 for each part of organic in the depolarizer. The cells of the present invention are particularly useful in electric watch, calculator and heart pacer applications.
U.S. Pat. Nos. 3,660,163 and 3,674,562 disclose batteries utilizing charge transfer complexes which are mixed with excess amounts of iodine. In particular, U.S. Pat. No. 3,674,562 teaches a novel cathode material which is "plastic" that is a pliable, putty-like solid. This material when used as a cathode in lithium primary cells has a low internal impedance and a relatively low self-discharge as measured by its heat of discharge. Cells manufactured in accordance with U.S. Pat. No. 3,674,562 are especially well suited for use in long-life, low current drains applications such as heart pacers.
What has been found is that 3-30 parts of iodine can be mixed with each part of the polymer P2VP or P2VQ having a weight average molecular weight* of above about 1.times.10.sup.3 and pelletized without becoming plastic. Unlike the "plastic" depolarizer, P2VP.nI.sub.2 and P2VQ.nI.sub.2, material of U.S. Pat. No. 3,674,562, the pelletized P2VP+P2VP.n.sub.1 I.sub.2 +n.sub.2 I.sub.2 (where n.sub.1 +n.sub.2 =3 to 30) depolarizer of the present invention has initially unacceptable electrical characteristics for battery applications. Typically, the initial impedance of a cell manufactured with the pelletized depolarizer of the present invention is quite high, for example, impedances from 2,000 ohms to 20,000 ohms are experienced. This compares with initial impedances of cells made with the material disclosed in U.S. Pat. No. 3,674,562 which have impedances in the range of about 30 to 300 ohms. FNT * Mw=.SIGMA.NiMi.sup.2 /.SIGMA.NiMi=.SIGMA.WiMi
It was found, however, that initial impedances greatly diminish with time so that the cell "acquires" the appropriate electrical characteristics for battery applications. In fact, cells made in accordance with the present invention result, after significant discharge, in impedances less than those made from the plastic depolarizer. In certain instances, the internal impedance in batteries of the present invention was less by a factor of five than those made with plastic depolarizers.
Generally, it has been well known to use various charge transfer materials, including P2VP and P2VQ, for complexing with halides, normally iodine, for use as depolarizers. Also, it has been equally well known that such materials could be pelletized for use in primary cells, U.S. Pat. Nos. 3,438,813 and 3,660,164. The depolarizer is each instance was in the form of a complex wherein the halogen comprises from 50 to 71% by weight of the complex. Cells produced in this manner had relatively short operating lives. Another proposed approach has been to utilize substantially pure iodine pellets having an additive of an electrically conductive material therein to which a coating of polyvinyl pyridine iodine is applied to the outer surface. U.S. Pat. No. 3,937,635. Typically, high portions of uncomplexed iodine increased the internal cell resistance. Pelletized cells having a charge transfer complex mixed with iodine in a ratio of 3 to 10 were found to have useful electrical properties. U.S. Pat. No. 3,660,163. The advantages of the greater iodine content, however, were achieved by forming the complex and iodine in the plastic state. See also U.S. Pat. No. 3,674,562.
It is, accordingly, an object of the present invention to provide a lithium halide battery in which the depolarizer is a pelletized particulate and comprises either P2VP+P2VP.n.sub.1 I.sub.2 +n.sub.2 I.sub.2 or P2VQ+P2VQ.n.sub.1 I.sub.2 +n.sub.2 I.sub.2 where n.sub.1 &gt;0 and n.sub.1 +n.sub.2 =3 to 30. The batteries made in accordance with the present invention have significant advantages over prior art batteries. These advantages will become apparent from a perusual of the following description of the invention taken in connection with the accompanying drawings.